1. Field of the Invention
This invention relates to phase transition-type liquid crystal compositions. In particular, this invention relates to cholesteric-nematic phase transition-type liquid crystal composition whose dielectric anisotropies are positive. Surprisingly, liquid crystal devices or elements produced using the liquid crystal composition of this invention may be effectively used in connection with keyboard inputting operations due to their East writing capabilities, and to provide large-scale or area display devices similar to those well-known in the art, such as cathode ray tube (CRT) display devices.
2. Description of the Related Art
A variety of X-Y dot matrix-type liquid crystal display devices based on different display modes have been proposed and used for display purposes. A conventional display modes is the dynamic scattering mode (DSM) which utilizes dynamic scattering effects. Liquid crystal display devices based on DSM, however, cannot be used in connection with large-scale displays. This is because, in large devices, cross-talk is undesirably caused with an increase of display area, and scanning lines, and there is a lower cut-off frequency.
Another display mode, mainly used in the field of liquid crystal displays, is the twisted nematic (TN) mode utilizing twisted nematic effects. However, as in the DSM-type devices, the TN-type liquid crystal devices suffer from cross-talk when they are used in a large-scale dot matrix display, because TN-type liquid crystals have no sudden rise characteristic. This means they are not suitable for large scaled displays. In addition to this, in a large-scale display, TN-type devices suffer from an unusually limited viewing angle and therefore have an unacceptably low contrast characteristic. Such narrowing of the viewing angle coupled with a need for an increase of the number of scanning lines is a critical drawback a need for in large-scale displays and must be overcome.
Numerous researchers have conducted experiments and studies in an effort to overcome the drawbacks of the above-discussed DSM-type and TN-type display devices Such work has led to the development of a new type of liquid crystal display device, i.e., a storage-type liquid crystal device characterized by being capable of maintaining written information or a display in a storage state after a single application of voltage and subsequent removal of the applied voltage (without further application of voltage) as a result of the memory effects of the selected specific liquid crystals. This phenomena enables realization of a large-scale display having as many as 500 or more scanning lines. Such large-scale display could not achieved with the conventional DSM-type and TN-type display devices of an X-Y dot matrix-type.
However, such storage-type liquid crystal display usually requires a driving voltage of .+-.20 V, an extremely high driving voltage for a liquid crystal display. When attempting to drive such storage-type liquid crystal display with a voltage in the range possible using a complementary metal oxide semiconductor (CMOS), even if writing is possible, essentially no storage time is obtained, making use of such storage-type liquid crystal display element in conjunction with a CMOS system impractical in a commercial sense.
For this reason, the above-mentioned storage-type liquid crystal display could not be driven using a CMOS integrated circuit (IC). Furthermore, while large scale dot matrix display have been possible in principle, because mounting of the driving circuit was difficult, large-scale dot matrix storage-type liquid crystal displays were unavailable for use in actual display devices.
Japanese Unexamined Patent Publication (Kokai) No. 50-159294 discloses a matrix-type liquid crystal device based on a nematic-cholesteric mixed liquid crystal display system, utilizing a Schiff-type cholesteric liquid crystal, for example, cholesteryl chloride. In such liquid crystal device the problem of cross-talk discussed above is avoided and increased contrast characteristics are provided in large-scale displays. However, such device has the drawback that in the case of an X-Y matrix display drive, when the liquid crystal is transferred to a storage state after the completion of the writing of picture, the entire picture becomes temporarily cloudy, e.g., for 10 seconds. The clouding of the entire picture means that the information on the display disappears, even if only temporarily, causing the operator to be uneasy. Furthermore, when the written-in picture is partially rewritten, the rewritten part becomes temporarily cloudy in the same manner each time, making the rewriting operation longer.
Under these circumstances, we found an improved storage-type liquid crystal composition and an improved X-Y dot matrix-type liquid crystal display device using such Liquid crystal composition. Japanese Unexamined Patent Publication (Kokai) No. 59-126492 and the equivalent U.S. application Ser. No. 558,475 filed Dec. 6, 1983, now U.S. Pat. No. 4,547,309 disclose the use of a storage-type liquid crystal composition incorporates a nematic-cholesteric mixture liquid crystal exhibiting a positive anisotropy of dielectric constant. The composition comprises a cyclohexane-type liquid crystal having a high anisotropy of dielectric constant (.DELTA..epsilon.) and a low viscosity (.eta.), as a nematic liquid crystal; an azoxy-type liquid crystal having an abrupt rising property; and a compensated cholesteric liquid crystal having a large spiral pitch and possessing the property of reducing the viscosity when admixed with the nematic liquid crystal and/or a cholesteric liquid crystal simply possessing the property of reducing the viscosity, such as, for example, cholesteryl oleate.
The storage-type liquid crystal composition of application Ser. No. 558,475 is effective to overcome many of the drawbacks of the above-discussed prior art liquid crystal compositions and devices. In fact, when such storage-type liquid crystal compositions are used, the necessary driving voltage is low, e.g., 7.0 V to 4.4 V, and no temporary clouding occurs. However, this liquid crystal composition is not completely satisfactory, since it can not provide a liquid crystal device having compatibility with the keyboard input system which are conventionally used for input in the display field.
Recently, we found a more improved storage-type liquid crystal display device using a new driving method (cf., for example, Japanese Patent Application No. 59-107404, filed May 29, 1984). The liquid crystal display device, as is disclosed such application, uses as the liquid crystal material a liquid crystal composition comprising 10% to 90% by weight of a cyanobiphenyl-type liquid crystal compound and 7% to 30% by weight of 4-cyano-4'-2-methylbutylbiphenyl-type liquid crystal compound. In such display device, two optical states, i.e., focal-conic or F state (cloudy state) and homeotropic or H' state (transparent state) can be produced and maintained With application of the same retention voltage. It has been found that such composition and such device are useful in connection with large size displays and employing a keyboard input system, provide a high contrast display, and result in the elimination of flicker. One drawback of such device is that stable homeotropic or H' state cannot be attained with application of the retention voltage. In fact, the H' state is metastable and therefore is changed to the focal-conic or F state after about 20 to 30 seconds. It is believed that such a change of state is caused because atmospheric conditions such as humidity, adversely affect the characteristics of the liquid crystal material. Another drawback is that the response speed of the device is relatively slow. It is therefore desirable to provide a still further improved novel liquid crystal composition having none of the drawbacks discussed above.